1. Field of the Invention
This invention relates to a method for absorbing acrylic acid and a method for purifying acrylic acid, and more particularly to a method for the purification of acrylic acid which comprises absorbing acrylic acid by contact thereof at a specific ratio with a high boiling inert hydrophobic organic liquid and purifying the high boiling inert hydrophobic organic liquid containing acrylic acid at a specific concentration
2. Description of the Related Art
Such easily polymerizing substances as acrylic acid and methacrylic acid are raw materials for commercial manufacture and are chemical substances which are mass produced in a plant of a large scale. Since these compounds are generally produced through various steps of absorption and purification for the purpose of affording manufactured goods of high purity.
When propylene, propane, or acrolein, for example, is subjected to catalytic gas phase oxidation with a molecular oxygen-containing gas in the presence of an oxidizing catalyst, the oxidation generates such low boiling substances as acetic acid, lower aldehydes, and water and such high boiling substances as furfural and maleic anhydride as by products in addition to acrylic acid which is a target product. Thus, the mixed gas consequently obtained is guided to an acrylic acid absorption column and brought into contact therein with an absorbent to absorb and collect acrylic acid into the liquid, thereby obtaining a solution containing acrylic acid and other by-products and this solution is subjected to distillation, stripping, extraction, or crystallization to separate and purify acrylic acid and the product aimed at is obtained.
Since the absorption of acrylic acid from the acrylic acid-containing gas allows the acrylic acid to be absorbed in a high concentration, generally an absorbent having water as a main component is used for this absorption. Since this absorption can be attained in a high absorbing coefficient even at a comparatively high temperature in the range of 50-70° C., it is at an advantage in permitting a saving in the energy needed to cool the reaction gas which is generated at a high temperature in the neighborhood of 300° C.
The step for removing low boiling substances, however, requires a large volume of energy because it entails the necessity for separating water. Further, since the aqueous absorbent, while permitting easy absorption of acrylic acid, contains acrylic acid which is an easily polymerizing compound at a high concentration, it is liable to form acid dimer and polymer at the steps of absorption and purification. Particularly, since the water itself is a low boiling substance, the acrylic acid gains in concentration after from the step for separation and purification in a light ends cut column and tends to generate a polymer. Further, the low boiling substances are not easily separated in a purified form from acrylic acid and are possibly suffered to persist in relevant manufactured goods to the extent of degrading the purity thereof.
In contrast to the method which uses such a low boiling substance as water, for example, as the absorbent, a method for absorbing acrylic acid by using a high boiling inert hydrophobic organic liquid has been also known. This method is at an advantage in not suffering acrylic acid to gain in concentration till the final step for obtaining acrylic acid as a final product and repressing the amounts of dimer and polymer to be formed because the absorbent is a high boiling substance. Further, the separation of low boiling substances and the consequent purification of acrylic acid are attained far easier in this method than the method using the aqueous absorbent. Thus, the practice of purifying acrylic acid via an operation of recovery using a high boiling inert hydrophobic organic liquid prevails.
The official gazette of U.S. Pat. No. 4,110,370, for example, gives a description to the effect that, in a method for separating acrylic acid from a reaction gas mixture resulting from the catalytic gas phase oxidation of propylene by counter-flow absorption using a high boiling inert hydrophobic organic liquid, a mixture consisting of 73.5 wt. % of diphenyl ether and 26.5 wt. % of diphenyl is supplied at a flow rate of 450 kg per hour to an absorption column and the reaction gas arising from catalytic oxidation of propylene containing 3 vol. % of acrylic acid is supplied at a flow rate of 520 Nm3 per hour to the absorption column to absorb the acrylic acid. This operation as reduced to the standard condition equals supply of an absorbent at a feed rate of 450 kg per hour to 50.2 kg (520 Nm3×0.03×72.1 g/0.0224 m3) of acrylic acid. Thus, the ratio of the amount of the absorbent relative to acrylic acid of the operation disclosed in the official gazette is 450 kg/50.2 kg=about 9 times the amount of a high boiling inert hydrophobic organic liquid.
Then, the official gazette of U.S. Pat. No. 3,932,500 discloses in a working example cited therein the technique of absorbing acrylic acid by supplying 2692 N liters of a reaction gas containing 2.52 vol. % of acrylic acid to an absorption column and introducing thereto an absorbent consisting of 75 wt. % of diphenyl ether and 25 wt. % of diphenyl at a flow rate of 1.98 kg per hour. This operation as reduced to the standard condition equals supply of 1.98 kg per hour of the absorbent to 218 g (2692 N liters×0.0252×72.1 g/22.4 liters) of acrylic acid. This time again, the ratio of the amount of the absorbent relative to acrylic acid of the operation disclosed in the official gazette is 1.98 kg/218 g=about 9 times the amount of a high boiling inert hydrophobic organic liquid.
The method disclosed in the official gazette of U.S. Pat. No. 5,780,679, likewise in a process of separating acrylic acid from the reaction gas mixture resulting from the catalytic gas phase oxidation of propylene and/or acrolein by counter-flow absorption using a high boiling inert hydrophobic organic liquid, absorbs acrylic acid by supplying an absorbent consisting of 57.2 weight % of diphenyl ether, 20.7 weight % of diphenyl, and 20 weight % of o-dimethyl phthalate at 52° C. at a flow rate of 160.8 kg per hour to 220.2 kg of a gas containing 13.5 weight % of acrylic acid at 204° C. The ratio of the amount of the absorbent relative to acrylic acid as disclosed in the official gazette is 160.8 kg/h/(220.2 kg/h×13.5/100)=5.4. This official gazette has a mention to the effect that in consequence of the absorption mentioned above, the bottoms of the acrylic acid absorption column have a low boiling substance concentration of 0.16 weight %.
When acrylic acid is absorbed by the use of a high boiling inert hydrophobic organic liquid as described above, since the organic liquid manifests an inferior absorbing ability to acrylic acid, it is ordinary to use the organic liquid in a large volume in an effort to repress the loss of acrylic acid through the top of the acrylic acid absorption column. In the conventional techniques, the ratios of the amount of an absorbent relative to acrylic acid have been invariably not less than 5.4 as described above.
Since the high boiling inert hydrophobic organic liquid has a high price, the necessity for reusing the organic liquid which has been used in the process for purifying the produced acrylic acid is markedly high as compared with the aqueous absorbent. In this case, since the repurification of the used organic liquid must avoid inducing excess concentration of the high boiling substance contained therein, the reuse of interest has its own limit. Further, the use of the organic liquid in a large amount inevitably entails the problem of adding to utilities for transfer and enlarging devices and pipes used for the transfer.
Moreover, the use of the high boiling inert, hydrophobic organic liquid particularly in a large amount as an absorbent entails an addition to the amount of absorption of the low boiling substance which is difficult of separation from acrylic acid. Since acrylic acid and the low boiling substance are separated only with difficulty in the distillation column, the low boiling substance possibly persists in the crude acrylic acid to be produced. For the purpose of decreasing the mass of the low boiling substance in the crude acrylic acid, therefore, it becomes necessary to subject the crude acrylic acid further to a treatment for purification intended to separate the low boiling substance and, to increase the reflux ratio during the course of the purification.